1. Field of the Invention
This invention relates to wet strength resin compositions for paper and a method for making them.
2. Description of the Related Art
Aminopolyamide-epichlorohydrin resins were the first commercially significant neutral-to-alkaline curing wet strength resins for paper. In addition to the health and safety benefits that result from the resins being formaldehyde-free, their use also affords such benefits as reduced paper-machine corrosion and improved softness and absorbency in treated webs compared to urea-formaldehyde-treated webs. U.S. Pat. No. 2,926,116 teaches high wet strength paper is obtained by treating paper pulp with a water-soluble polymeric reaction product of epichlorohydrin and a polyamide derived from a polyalkylenepolyamine and a dicarboxylic acid having from 3 to 10 carbon atoms. Other patents teaching the preparation and/or use of aminopolyamide-epichlorohydrin resins in wet strength applications for paper include U.S. Pat. Nos. 2,926,154; 3,049,469; 3,058,873; 3,066,066; 3,125,552; 3,186,900; 3,197,427; 3,224,986; 3,224,990; 3,227,615; 3,227,671; 3,239,491; 3,240,761; 3,248,280; 3,250,664; 3,311,594; 3,329,657; 3,332,834; 3,332,901; 3,352,833; 3,248,280; 3,442,754; 3,459,697; 3,483,077; 3,609,126; 4,714,736; British patents 1,073,444 and 1,218,394; Finnish patent 36,237 (CA 65: 50543d); French patent 1,522,583 (CA 71:82835d); German patents 1,906,561 (CA 72: 45235h); 2,938,588 (CA 95: 9046t); 3,323,732 (CA 102: 151160c); Japanese patents 70 27,833 (CA 74: 4182m); 71 08,875 (CA 75: 49990k); 71 12,083 (CA 76: 115106a); 71, 12,088 (CA 76: 115107b); 71 36,485 (CA 77: 90336f); Netherlands application 6,410,230 (CA 63: P5858h); South African patent 68 05,823 (CA 71: 114420h); Swedish patent 210,023 (CA 70: 20755y).
One of the drawbacks associated with the use of aminopolyamide-epichlorohydrin wet strength resins is the emission of harmful chlorinated compounds into the water systems of pulp and paper mills. These chlorinated compounds, which are the by-products of the manufacture of the aminopolyamide-epichlorohydrin resins, have been identified as epichlorohydrin, 1,3-dichloro-2-propanol, and 3-chloro-1,2-propanediol. These chlorinated organics are usually discharged into effluent waste water systems from pulp and paper mills because they are not substantive to cellulose pulp fibers. Since permissible amounts of halogenated organics in waste waters is ever decreasing, considerable effort has been expended to reduce the amount of these materials in aminopolyamide-epichlorohydrin wet strength resins. For example, European Patent Application 335,158 teaches that wet strength resins having organic chlorine values of about 0.001% are obtained by reaction of an aminopolyamide with a glycidyl sulfonate. European Patent Application 332,967 teaches that wet strength compositions containing 15% by weight of aminopolyamide-epichlorohydrin resins having organic chlorine contents of about 0.16% by weight of total composition or 1.1% by weight of aminopolyamide-epichlorohydrin acid salt resin are obtained by adding epichlorohydrin to an aqueous solution of aminopolyamide at 25.degree. C.-30.degree. C. and then heating the reaction mixture to 75.degree. C. for a period of time sufficient to increase the viscosity of a 15% solids solution to 55 MPa-s at 25.degree. C. The moles of epichlorohydrin/amine equivalents weight (E/N, infra) of the composition is equal to about 1.0 European Patent Application 332,967 teaches an elevated epichlorohydrin reaction temperature of 75.degree. C. as compared to from about 15.degree. C. to about 25.degree. C. according to the invention and an E/N ratio of about 1.0 as compared to from about 0.6 to about 0.8 according to the invention. The application does not teach or suggest that very low total organic chlorine levels are obtainable by lowering the epichlorohydrin reaction temperature and lowering the E/N ratio. European Patent Application 335,157 teaches an epichlorohydrin reaction temperature of 55.degree. C. as compared to from about 15.degree. C. to about 25.degree. C. according to the invention and an E/N ratio of about 1.2 as compared to from about 0.6 to about 0.8 according to the invention. The composition produced by the disclosed process contains about 3.12% total organic chlorine based upon the weight of the aminopolyamide-epichlorohydrin acid salt resin. The application does not teach or suggest that very low total organic chlorine levels are obtainable by lowering the epichlorohydrin reaction temperature and lowering the E/N ratio. Wochenblatt fur Papierfabrikation 2, 63-67 (1989) teaches that a modified aminopolyamide-epichlorohydrin resin has an absorbable organic halogens content 70% lower than conventional commercial products. The wet strength resin compositions according to the invention contain organic chlorine compounds in amounts which are at least 99% lower than commercial aminopolyamide-epichlorohydrin resins. Wochenblatt fur Papierfabrikation 2, 63-67 (1989) neither identifies the modified aminopolyamide-epichlorohydrin resin nor teaches how to make it.
The related art does not teach wet strength resins compositions comprised of an aminopolyamide-epichlorohydrin acid salt resin and up to about 0.1% by weight total organic chlorine based on the weight of said resin. The related art also does not teach a relationship between the epichlorohydrin-aminopolyamide reaction time and very low total organic chlorine content in processes to make wet strength resins compositions comprised of an aminopolyamide-epichlorohydrin acid salt resins.